1. Field of the Invention
This invention relates to honeycomb structures and, in particular, to honeycomb structures which can be heated electrically.
2. Description of the Prior Art
Electrically heatable honeycomb structures of various configurations have been proposed in the art to serve as catalytic converters, preheaters for catalytic converters, and particulate filters.
For example, Japanese Patent Publication No. 57-110,311 discloses a filter for diesel exhaust gases formed by sealing off alternate cells on the two ends of a porous honeycomb. (See Pitcher, Jr., U.S. Pat. No. 4,329,162, for the basic structure of such a particulate filter.) The filter of the Japanese publication is composed of silicon carbide so that it is electrically conductive. Accumulated carbon particles are burned out of the filter by passing electrical current through the honeycomb in the direction of the longitudinal axis of the cells. Japanese Patent Publication No. 58-143,817 discloses a similar silicon carbide filter in which the current is passed through the honeycomb in a direction transverse to the cells' longitudinal axis rather than parallel to that axis.
Japanese Patent Publication No. 58-143,815 discloses another approach for using an electrical current to regenerate a particulate filter. In this case, the filter is made of a non-conductive, porous ceramic and has an accordion rather than a honeycomb configuration. Accumulated carbon particles are burned off of this filter by passing current through the carbon particles rather than through the filter material.
With regard to catalytic converters, Mizuno et al., U.S. Pat. No. 5,063,029, discloses a metal honeycomb which is electrically heated and serves as a preheater for a conventional catalytic converter. Slits are cut into the honeycomb and filled with an insulating adhesive to increase the honeycomb's resistance by causing the current to follow an elongated serpentine path. Along these same lines, PCT Patent Publication No. WO89/10471 discloses a catalyst support comprising an electrically conductive honeycomb in which a serpentine current path is used to increase the honeycomb's electrical resistance.